1. Field of the Invention
The present invention relates to determining a back emf voltage of a hard disk drive voice coil motor.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
Disk drives typically employ a servo to maintain the heads on the centers of the tracks. Deviations from the track center line may create errors in the reading and/or writing of data. The servo utilizes servo bits written into servo fields of the disk. The servo bits are used to produce a position error signal (“PES”) that is processed to center the head on the disk track.
The drive may be subjected to vibration. For example, disk drives have a spindle motor that rotates the disks relative to the heads. Rotation of the spindle motor may create a vibration that is transmitted through the drive. Servo routines are typically constructed to compensate for vibration through detection and/or prediction algorithms. It is generally desirable to study vibration of a disk drive to create effective servo routines. One way of studying vibration is to analyze the read signal and/or PES of the disk drive. For various reasons it is also desirable to study vibration without servo, in an open loop system.
One way to detect and analyze vibration is by measuring and analyzing the back emf voltage of the voice coil motor. Voice coil motors are typically biased to a middle location (“MD”) of the disk. The head can be placed at the MD position and the back emf voltage can be measured and analyzed. It is desirable to study vibration while the heads are at any location across the stroke from disk inner diameter (“ID”) and outer diameter (“OD)”. This requires some type of force to maintain the heads at the specific location to balance with the bias forces due to air flow, FPC and pivot friction. For analysis of high frequency vibration the voice coil motor may be biased with a piece of tape. When analyzing low frequency vibration it has been found that the tape distorts the back emf values.